A conventional electrophotographic image forming apparatus detachably accommodates developing devices therein. Developing devices that are detachably mounted in this type of printer include a case for accommodating toner, a developing roller supported in this case for carrying toner on its peripheral surface, and a thickness-regulating blade that regulates a thickness of toner carried on the peripheral surface of the developing roller. The developing roller and thickness-regulating blade are arranged in this developing device at positions near an opening formed in the case. Consequently, it is necessary to prevent toner from leaking out of the case through gaps between the case and developing roller and between the case and thickness-regulating blade.
To prevent such toner leakage, there was proposed a developing device provided with a lower film, and an upper seal for preventing toner from leaking out through gaps between the case and other components of the developing device (for example, see Japanese patent application publication no. 2007-188109). This developing device is configured such that the lower film presses against the developing roller to suppress toner leakage between the developing roller and the case, while the upper seal presses against the thickness-regulating blade to suppress toner leakage between the thickness-regulating blade and the case.